The type subspecies B. m. musculus refers at least to the North Atlantic Blue Whale which was the basis for the first description by Linnaeus. Animals in the northern North Pacific are similar in size and morphology to North Atlantic Blue Whales and are also regarded as B. m. musculus. The Antarctic form B. m. intermedia Burmeister, 1871, sometimes called the "true" Blue Whale, is distinguished by its large body size and Antarctic distribution in summer. The pygmy blue whale B. m. brevicauda Ichihara, 1966 has a number of morphological characteristics that distinguish it fromB. m. intermedia and B. m. musculus, including the characteristic "tadpole" body shape (Kato et al. 2002). It occurs in the southern Indian Ocean, excluding the Antarctic, from Africa and Madagascar across to Indonesia, Australia and Tasmania. Blue Whales in the northern Indian Ocean have been assigned the name B. m. indica (Blyth, 1859), although Mikhalev (1996) regarded them as Pygmy Blue Whales and not significantly different from Pygmy Blue Whales in the southern Indian Ocean.

Blue Whales in the eastern Pacific from California in the north to 44°S in southern Chile have been considered similar to Pygmy Blue Whales (those off California and Baja California; Gilpatrick et al1997) or to the Antarctic form (off the Galapagos; Palacios 1999), while off Peru both Pygmy and Antarctic Blues have been reported (Donovan 1984, Van Waerebeek et al. 1997). High abundance in summer and lack of sightings south of 44°S suggest that the South American whales are not Antarctic blue whales, but some whales caught were larger than the maximum size of pygmy blue whales from the southern Indian Ocean (Branch et al. 2006). LeDuc et al. (2007) found that Blue Whales off western South America and off Western Australia differed genetically from each other as much as from Antarctic Blue Whales. No diagnostic genetic markers that distinguish the subspecies or populations of Blue Whales have been found to date, but that does not rule out that such might eventually be found.

In view of these uncertainties, the tendency to try to categorize all Blue Whales dichotomously into "true" and Pygmy Blue Whales may not be appropriate. Pending further elucidation of the subspecific taxonomy of Blue Whales, it is preferable to limit the term "Pygmy Blue Whale" to the Indian Ocean populations, and to use geographical names such as Antarctic Blue Whale and North Atlantic Blue Whale instead of "True Blue Whale". In the Pacific (outside the Antarctic), the subspecific taxonomy and nomenclature of Blue Whales should be considered open until more genetic and morphological data from more locations are available.

Justification:
The cause of the population reduction in this species (commercial whaling) is reversible, understood, and is not currently in operation. For this reason, the species is assessed under criterion A1, not under A2, A3 or A4. There is no doubt that the global blue whale population has been depleted greatly. Although there are uncertainties over present abundance, the total population has been depleted by at least 70%, and possibly as much as 90%, over the last three generations, assuming a 31-year average generation time. The species therefore meets the criterion A1(abd) for Endangered, and probably meets the same criterion for Critically Endangered. The dominant contribution to the reduction in the global population is the massive reduction of the formerly very large Antarctic population. For that reason, the Antarctic blue whale (B. m. intermedia) subspecies should be separately as Critically Endangered due to a reduction over the same period of over 97% (that assessment will proceed in future). The pygmy blue whale (B. m. brevicauda) subspecies is less depleted. It is included in this global assessment.

The blue whale is a cosmopolitan species, found in all oceans except the Arctic, but absent from some regional seas such as the Mediterranean, Okhotsk and Bering seas.

The Antarctic form B. m. intermedia, which used to be by far the most abundant form of blue whale, occurs in the Antarctic in summer, from the Antarctic Polar Front up to and into the ice (Branch et al. 2006), including (in the past) the South Georgia area. Its winter distribution is poorly known, but the presumption has been that animals migrate in winter to lower latitudes, largely because blue whales were caught off Namibia, South Africa and Chile in winter (Best 1998, Mackintosh 1965).

Pygmy blue whales (B. m. brevicauda) are confined mainly to the area north of 55°S even in summer, but with one record at 56°15'S (Ichihara 1966). They are most abundant in the southern Indian Ocean on the Madagascar plateau, and off South Australia and Western Australia, where they form part of a more or less continuous distribution from Tasmania to Indonesia. Blue whales are found year round in the northern and equatorial Indian Ocean, especially around Sri Lanka, and the Maldives, and at least seasonally near the Seychelles and in the Gulf of Aden.

Blue whales occur in the eastern Pacific from around 44°S in southern Chile (Hucke-Gaete et al. 2005) as far as the Costa Rica Dome where they are present year-round (Reilly and Thayer 1990). There may be a gap from there to Baja California where they are quite common as also off the Californian coast (Calambokidis and Barlow 2004) but tracking of a tagged whale suggests that some of the Californian whales may migrate to the Costa Rica Dome in winter (Mate et al. 1999). North of 40°N, blue whales occur across the North Pacific from the coast of Oregon to the Kurile Islands (Russian Federation), and north to the Aleutian Islands (US -Alaska) but not far into the Bering Sea. In the past blue whales were caught off southern Japan and the Korean peninsula, but none have been seen there in recent years.

In the North Atlantic the summer distribution of blue whales extends in the west from the Scotian Shelf to the Davis Strait (Canada) (NMFS 1998). Blue whales occur in the Denmark Strait, around Iceland and north to the ice edge, and in the northeast to Svalbard (Norway). Historically, blue whales were commonly caught along the coasts of North and West Norway, the Faeroes and the NW British Isles. They also occur in low numbers off NW Spain (Bérubé and Aguilar 1998) and in the past near the Strait of Gibraltar, but not in the Mediterranean (Reeves and Notarbartolo di Sciara 2006). The winter distribution is poorly known but it appears that in the past blue whales were widely distributed in the southern half of the North Atlantic in winter (Reeves et al. 2004).

McDonald et al. (2006) use song to suggest nine different groupings of blue whales. They argue that because song is used in mating, that these different song types, five of which have data spanning over 30 years and showing stability, should form the basis for population structure hypotheses. Although some of the geographic locations correspond to IWC stocks, for example the northern Indian Ocean, others do not. Thus, the population structure in this account likely underestimates the true number of discreet groups of blue whales.

Generation time. The inferred value of 31 years for generation time given in Taylor et al. (2007) is considered appropriate, given an absence of any indications to the contrary from available biological information for the species. That implies that the three generation time window for applying the A (past reduction) criterion is 1914-2007.

North AtlanticIn the North Atlantic, about 400 whales have been photo-identified in the Gulf of St Lawrence (Ramp et al. 2006) and Pike et al. (2004) estimate 1,000-2,000 in the central North Atlantic (Iceland, Denmark Strait, East Greenland, Jan Mayen, Faeroes and the British Isles). Sightings of blue whales are still very rare in areas where substantial catches were made in the past e.g. off Norway and especially in northern Norway (Christensen et al. 1992, Norwegian sighting surveys 1995-2006), Svalbard and the British Isles. Approximately 8,000 blue whales are specifically recorded in whaling statistics since the start of modern whaling in northern Norway in 1868, but an additional 30,000 unspecified large whales were recorded caught in the late 19th and early 20th centuries, of which perhaps as many as 25% could have been blue whales (IWC 2006). However, only about 1,600 blue whales were caught after 1914, hence the main decline occurred primarily before the time window of interest (three generations). The population is estimated to have been recovering at 5.2% p.a. (SE 1.1%) in the Iceland/Denmark Strait area during 1969-88, after catching had ceased (Sigurjónsson and Gunnlaugsson 1990). Taken together this all suggests that the North Atlantic population was very low when whaling ceased in the mid-1960s (apart from a very few pirate whaling catches up to 1978) and may now be at or above the 1911 level but still well below the pre-whaling level.

AntarcticThe Antarctic blue whale B. m. intermedia was extremely abundant in the past: about 341,830 blue whales have been recorded caught in the Antarctic and sub-Antarctic (IWC 2006) in the 20th century, of which 12,618 were identified as pygmy blue whales or are assumed to have been so from their location (Branch et al. 2004). About 40,000 of these were taken around South Georgia. In addition, the majority of the over 17,000 blue whales caught off southern Africa were probably Antarctic blue whales (Branch et al. 2006). Ignoring these and other catches north of 40°S, Branch et al. (2004) estimated the pre-exploitation (1905) abundance to be 239,000 (202,000-311,000). The current population size in 1996, based primarily on data from the IWC-sponsored whale sightings cruises conducted during 1978-2001, was estimated to be 1,700 (860-2,900) and to be increasing at the rate of 7.3% (1.4-11.6%) p.a. Branch et al.'s initial (1905) population estimate can be taken as a conservative proxy for the 1911 population size, because few (Southern Indian OceanNo precise estimates are available of the population of pygmy blue whales (B. m. brevicauda) in the southern Indian Ocean. From a survey in December 1996, Best et al. (2003) estimated the abundance of pygmy blue whales in a survey area south of Madagascar to be 424 with wide confidence limits (about 190-930) and suggested, based on the distribution of past catches, that the total population in the southwestern Indian Ocean may be about 3 times that in the survey area. Blue whales appear to be rare in the central southern Indian Ocean (Branch et al. 2006). They occur in the southeastern Indian Ocean off western and southern Australia but are abundant only in quite small areas (Kato et al. 1996, Bannister et al. 2007, Gill 2002), suggesting a population only in the hundreds. The catch of at least 12,618 pygmy blue whales in the southern Indian Ocean in a rather short period during 1960-71 (Branch et al., 2004) suggests that the initial population was at least this size, and hence that the current population is still depleted, but not as severely as the Antarctic blue whale.

Northern Indian OceanNo population estimates are available but blue whales are regularly observed off Sri Lanka (Alling et al. 1991) and the Maldives (Anderson 2005). Mikhalev (1996) reports 1,294 pygmy blue whales caught illegally by Soviet fleets during 1963-66, mainly off the Seychelles, the Maldives, in the Gulf of Aden, and west of southern India and Sri Lanka.

Western North PacificNo quantitative abundance estimates for western North Pacific blue whales are available. Japanese scouting surveys recorded 183 blue whales during about 165,000 nmi of search effort in the North Pacific north of 40°N in summer, 1974-2005, spread fairly uniformly throughout the area, although none were observed in coastal waters off Japan where they were hunted historically (Japanese Progress Reports to the IWC, 1975-2006; Clapham et al. 2008).

The IWC data tables list 7,300 blue whales caught in the North Pacific in the 20th Century (western and eastern), but to these should be added about 700 blue whales caught by Soviet fleets in the 1960s that were not reported at the time (Doroshenko 2000). In addition, about 20,000 unspecified large whales were caught during 1900-1930, of which an unknown proportion would have been blue whales (IWC 2006). About 1,500 were taken during the first half of the 20th century off southern Japan to Taiwan and Korea where no blue whales have been seen in recent times (Clapham et al. 2008).

Eastern North PacificFor blue whales in the eastern North Pacific (sub-specific identity uncertain), available population estimates are ~3,000 for the area off California and Baja California (Calambokidis and Barlow 2004). Some proportion of ~1,400 whales from a study that spans the equator and runs from late July through early December are from the North Pacific (Wade and Gerrodette 1993), although there may have been some double counting of whales censused off California and Baja California (Calamblokidis and Barlow 2004). The locations of the American pelagic catches are not all recorded, but up to 2,000 of the recorded blue whale catches and an unknown proportion of the unspecified catches could have been from the California-Mexican blue whale population.

Eastern South Pacific

Some proportion of ~1,400 whales from a study that spans the equator and runs from late July through early December are from the South Pacific (Wade and Gerrodette 1993). Donovan (1984) does not provide an abundance estimate from a survey of Peruvian and Ecuadorian waters, but on the assumption of a similar effective sighting distance to that calculated by Best et al. (2003) using the same vessel and similar procedures, an abundance of the order of ~1,000 whales is implied. No abundance estimates for Chile are available, but the fact that a blue whale fishery catching several hundred animals per year continued until its closure in 1967 without obvious signs of decline (371 blue whales being taken in 1965 alone) suggests a population in the thousands. No abundance estimate has yet been calculated from the IWC Blue Whale Survey in Chilean waters (Findlay et al.1998) but their sighting rate (~5 blue whales per 1,000 km) is consistent with a population in the low thousands. The survey missed a newly discovered blue whale summer feeding and nursery ground around Chiloé Island (41°-44°S) (Hucke-Gaete et al. 2005), which appears to contain a population at least in the hundreds.

Other areasRecent records of blue whales are very rare from the South Atlantic. A stranding at 34°S in southern Brazil could not be diagnosed unambiguously as a pygmy or "true" blue whale (Dalla Rosa and Secchi 1997). There are no records for the offshore central South Pacific outside the Antarctic, although data for this area are sparse (Branch et al. 2006).

GlobalThe global population of blue whales is uncertain, but based on the above information, the global total for the species is plausibly in the range 10,000-25,000, corresponding to about 3-11% of the 1911 population size.

Blue whales feed almost exclusively on euphausiids (krill), with a variety of species being taken by different blue whale populations, such as Euphausia superba in the Antarctic, Nyctiphanes australis off southern Australia (Gill 2002), Euphausia recurva off Western Australia (J. Bannister pers. comm. 2007) and Nyctiphanes simplex off the Galápagos (Palacios 1999). They feed both at the surface and also at depth, following the diurnal vertical migrations of their prey to at least 100 m (Sears 2002).

The migration patterns of blue whales are not well understood, but appear to be highly diverse. Some populations appear to be resident year-round in habitats of year-round high productivity, while others undertake long migrations to high-latitude feeding grounds (see above), but the extent of migrations and the components of the populations that undertake them are poorly known.

The main threat in the past was direct exploitation, which only became possible in the modern era using deck-mounted harpoon cannons. Blue whale hunting started in the North Atlantic in 1868 and spread to other regions around 1900 after the northeastern Atlantic populations had been severely reduced. The Antarctic and North Atlantic populations were probably depleted to the low hundreds by the time whaling ceased, but are increasing (see above). Blue whales have been protected worldwide since 1966, although they continued to be caught illegally by former USSR fleets until 1972. The last recorded deliberate catches were off Spain in 1978 (IWC 2006).

Blue whales are subject to some ship strikes and entanglements (NMFS 1998) but reported cases are few. The remote distribution of some blue whale populations probably makes them less vulnerable to human impacts than some other cetacean species, but local populations that inhabit waters with significant levels of human activity may be subject to some threat, such as disturbance from vessel traffic, including ship noise (e.g. Gulf of St Lawrence population, NMFS 1998). Globally, there appear to be no major threats to blue whales at present.

During this century, a profound reduction in the extent of sea ice in the Antarctic is expected, and possibly a complete disappearance in summer, as mean Antarctic temperatures rise faster than the global average (Turner et al. 2006). The implications of this for blue whales are unclear but warrant monitoring.

Small populations such as the surviving Antarctic population can have a number of interacting effects that accelerate overall risk (Gilpin and Soule, 1986). Among those effects are demographic stochasticity, inbreeding depression and density dispensation (Allee effects). Although the expectation is that these threats could be serious because cetaceans are social animals with low reproductive output, the fact that the Antarctic population is increasing is encouraging.

The IWC had granted protection to blue whales by 1966. Catch limits for all commercial whaling have been set at zero by the IWC since 1986. However, this moratorium does not apply to Iceland, Norway or the Russian Federation, which have objected to this provision. No blue whales have been recorded deliberately caught since 1978. The species is on Appendix I of both CITES and CMS.

Local measures may be required to protect the habitat of specific local populations in order to ensure their long-term viability in the face of increasing human impacts, e.g. see Hucke-Gaete et al. 2005.

In-Place Research, Monitoring and PlanningIn-Place Land/Water Protection and ManagementIn-Place Species ManagementIn-Place Education Included in international legislation:Yes Subject to any international management/trade controls:Yes

Sigurjonsson, J. and Gunnlaugsson, T. 1990. Recent trends in abundance of blue (Balaenoptera musculus) and humpback whales (Megaptera novaeangliae) off West and Southwest Iceland, with a note on occurrence of other cetacean species. Report of the International Whaling Commission 40: 537-551.